This invention relates to magnetic recording media, and more particularly, to magnetic recording media having improved physical properties, shelf stability, runnability, and electromagnetic properties in a high temperature, high humidity environment.
Vinyl chloride-vinyl acetate-vinyl alcohol copolymers have been widely used as the binder resin for coating type magnetic tape. Typical products of vinyl chloride-vinyl acetate-vinyl alcohol copolymer are commercially available under the trade mark "VAGH" series from UCC of the U.S. These copolymers have many advantages including solubility in various solvents, compatibility with other resins, and affinity to and hence, dispersion with magnetic powder due to their hydroxyl group. Particularly, the enhanced dispersion of magnetic powder in the copolymer ensures that the resulting magnetic coating composition exhibits good magnetic properties such as degree of orientation and maximum residual magnetic flux density as well as good electromagnetic properties.
In recent years, the magnetic powder is available in more finely divided form as will be described later. The conventional resins are not necessarily satisfactory in dispersion of such more finely divided magnetic powder. Attempts were made to promote dispersion of finely divided magnetic powder in a copolymer of this type by increasing the content of hydroxyl group in the copolymer or by copolymerizing a monomer copolymerizable with vinyl chloride, for example, maleic acid or its anhydride. Copolymers having an increased content of hydroxyl group and copolymers having copolymerized a monomer copolymerizable with vinyl chloride, however, tend to release hydrogen chloride or degrade in a similar way because they have experienced saponification for hydrolysis during their preparation process. Thus these tailored copolymers are less stable to heat so that the resulting magnetic recording media are insufficient in physical properties, an inter alia in shelf stability and runnability (or durability during repeated operation), particularly in a high temperature, high humidity environment.
Vinyl chloride-vinyl acetate-maleic acid copolymers which have not been subjected to saponification and possess a polar group-containing monomer are commercially available under the trade mark "VMCH" series from UCC of the U.S. These copolymers are inferior in dispersion and physical properties to the above-mentioned quaternary copolymers of vinyl chloride-vinyl acetate-vinyl alcohol-maleic acid having been subjected to saponification during their preparation, and thus unsuitable in dispersing the advanced micronized magnetic powder.
For the purpose of improving the magnetic properties of magnetic recording media, there is the trend of converting the magnetic powder from conventional oxide powders to metal magnetic powders for providing a higher recording density. At the same time, the magnetic powder is more micronized. Particularly in audio applications, micronizing of metal magnetic powder is essentially required to reduce noise for S/N ratio improvement. It was also found that the use of alkaline metal magnetic power affords advantageous electromagnetic properties.
Micronized metal magnetic powder is, however, difficult to uniformly disperse in a magnetic coating composition. It is thus difficult to provide the magnetic layer with a high content of magnetic powder. The micronized magnetic powder itself is more reactive than the conventional magnetic powder, and thus susceptible to a change with time in a high temperature, high humidity environment and less reliable or less stable in physical properties.
There still remains the need for further improving the dispersion and thermal stability of a coating composition of micronized magnetic powder.